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Dropbox does this too. I think I read somewhere that they md5 your file and see if matches one already on the network, if so, they just replicate a 'link' to that file in your account.

That's why sometimes if you upload a movie you downloaded, it uploads in a matter of seconds.

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I thought they changed that because someone found that if you added that hash to your Dropbox to your local Dropbox database/config, it would download the movie. And for a week or two, people were pirating movies this way.

Though I suppose Dropbox could still use the same deduplication while making it harder to link a file to your account.

I believe they broke that scheme using other methods, which I can't recall.

Edit: They changed how it works on the server side, without going into detail. It's discussed here: https://news.ycombinator.com/item?id=2483053

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